"Freedom is not always about doing what you want. In fact it is the gift of being able to choose the right way," he said in a homily punctuated by regular bursts of applause from the crowd on a packed St Peter's Square.

"Your happiness has no price. It cannot be bought and sold: it is not an application you download on a mobile phone. Even the latest version cannot help you to grow and become free in love."

An estimated 70,000 teenagers were in Rome for a weekend of events to celebrate Francis's Jubilee year dedicated to the theme of mercy.

In a surprise move on Saturday, the 79-year-old pontiff heard confessions from 16 of them and a video message from him was broadcast at a rock and rap concert in the Stadio Olimpico.

One of the teenagers chosen to confess to the leader of the world's 1.2 billion Catholics said she had been trembling as she approached the chair on which the pope was sitting in St Peter's Square.

"But as soon as I sat down I had the feeling of being next to a normal person rather than the pope. Francis is really one of us," Anna Taibi, 15, told La Repubblica newspaper.

The Sicilian teenager said she had been touched by Francis's tenderness as he listened to her confession.